Retroviruses can be oncogenic or function as safe gene transfer vectors, but all retroviruses utilize host cell proteins to achieve cytoplasmic expression of their unspliced genomic RNA. We have identified that the 5' LTR of spleen necrosis virus (SNV) facilitates Rev-independent expression of HIV-1 unspticed reporter RNA. The RU5 region of SNV is a distinct position- and orientation-dependent translational enhancer of introncontaining HIV-1 gag reporter RNA and also nonviral RNA. Polysome analyses have shown that the functional mechanism is to increase polysome association, but SNV RU5 is not an internal ribosome entry site. Recently, comparable activity was identified in the 5' terminal sequences of two other retroviruses. An attractive model is that 5' cap-dependent translational enhancers are a feature shared among divergent retroviruses and selected cellular RNAs to modulate translational efficiency,. Our long-term goal is to characterize virus-host interactions that mediate translational enhancement by retroviral and cellular 5' posttranscdptional control elements. Herein, we will test the hypothesis that SNV RU5 overcomes barriers to efficient translation by recruitment of host nuclear-cytoplasmic shuttle protein(s). We postulate that PCE functions in place of the 5' exon-exon junction to recruit RNP components that are necessary for efficient cytoplasmic expression, in addition, we will apply our new mechanistic findings to develop translational control as an innovative switch to optimize retroviral vectors for therapeutic gene delivery. We implement translational repression in helper cells to augment retroviral vector packaging and titer, and translational enhancement by PCE in transduced target cells to optimize vector transgene expression. Our three integrated specific aims will evaluate PCE structure, function, and application to innovative novel retroviral vectors. Our results will illuminate mechanisms that control eukaryotic RNA expression and virus-host interactions that are important for viral replication and progression to disease. Our specific aims are: 1) To characterize the structure-function relationship of PCE. 2) To define the role of retroviral 5' UTR and intronic sequences on PCE activity. 3) To apply our mechanistic findings of translational control to optimize the efficiency of lentiviral transduction.